Thermally stable insulating oil

ABSTRACT

A thermally stable insulating oil comprising from 95 to 20 percent by volume of a polycyclic naphthenic hydrocarbon and from 5 to 80 percent by volume of a member selected from the group consisting of a polycyclic aromatic hydrocarbon, its lower alkyl derivative and a mixture thereof.

United States Patent m1 m1 3,714,021

Takahashi et al. 1 Jan. 30, 1973 [54] THERMALLY STABLE INSULATING [56]References Cited UNITED STATES PATENTS [75] inventors: MasaakiTakahashl, Tokyo-to;

Takashi Yamauchi, TOkyo; Kens k 3,252,887 5/1966 Rizzuti ..208/l4 OkudaAkira Ito Tokyo 3" H Wynkoop el al. f Japan 3,462,358 8/1969 Mills et al..2os |4 2,846,372 8/1958 Schneider et al..... ..208/l4 1 AsslgneerKureha Kflgak" Kogyo Kabushlkl 3,095,366 6/]963 Schieman ..20s/19Kaisha, Tokyo-to, Japan 22 pn Oct 22, 1970 Primary Examiner-HerbertLevine 1 pp No-183,232 Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak57 ABSTRACT [30] Foreign Application Pnomy Data A thermally stableinsulating oil comprising from 95 to Oct. 22, 1969 Japan ..44/84060 20percent by volume of a polycyclic naphthenic hydrocarbon and from 5 to80 percent by volume of a U-S. .i member eected from the groupconsisting of a polya r 208/ cyclic aromatic hydrocarbon, its loweralkyl derivative Int. Cl. ..Cl0g 37/06 and a mixture thereof 1 Field ofSearch ..208/l4, l9

18 Claims, No Drawings TI'IERMALLY STABLE INSULATING OIL BACKGROUND OFTHE INVENTION 1. Field of the Invention The present invention relates toan insulating oil having superior thermal stability, and moreparticularly, it relates to a novel electrical insulating oil comprisinga mixture of a polycyclic naphthenic hydrocarbon having two or morerings in the molecule and a polycyclic aromatic hydrocarbon having twoor more rings in the molecule.

2. Description of the Prior Art Conventional mineral oil-type insulatingoils used hitherto as transformer oils, condenser oils, cable oils, andthe like electrical insulating oils, have been prepared frommiddle-distilled fractions of crude oil after purification thereof byvarious treatments. Therefore, the performance of the resulting oilvaries over a wide range depending upon the kind and nature of the crudeoil used, so that the choice of a suitable crude oil has been amatter ofgreat importance in the produc-- tion of insulating oils. 1

These known mineral oils, however, have limited thermal stability,,i.e., they have a maximum usable temperature of at most about 100C.Many difficulties have often been encountered when using these mineraloil-type insulating oilsvin transformers, condensers, ca-

bles and like electrical equipment wherethey are used at a temperatureexceeding the above maximum tem-' perature. It is of course possible toimprove' the thermal stability to a certain extent by resorting to theaddition of an oxidation inhibitor and similar additives, but this israther impractical because the performance of the insulating oilabruptly decreases when the oxidation inhibitor has been used up.

The present invention is based on the discovery that the gasabsorption'performanceof an insulating oil is unexpectedly improved,more so than any other electrical characteristic, and thethermalstability in the presence of oxygen is also greatly enhanced by admixinga polycyclic aromatic hydrocarbon, having two or more rings in themolecule, to a'particular hydrocarbon mixture comprising polycyclicnaphthenes having two or more rings in the molecule. v

The primary object of the present invention is to provide an electricalinsulating oil having improved properties.

SUMMARY OF THE INVENTION The present invention provides an electricalinsulating oil comprising a mixture of from 95- to 20 percent, byvolume, of a polycyclic naphthenic hydrocarbonof two or more rings oralkyl derivatives thereof or a mixture thereof, and 5 to 80 percent, byvolume, of a polycyclic aromatic hydrocarbon of two or more rings oralkyl derivatives thereof or a mixture thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The polycyclic naphthenichydrocarbons which may be used in this invention include, for example,decaline, perhydrophenanthrene, perhydropyrene, perhydrofluorene,perhydroanthracene, perhydrochrysene, and like cyclic naphthenes havingtwo i to five rings, or C or lower, preferably C or lower,

alkyl derivatives thereof or mixtures of two or more such hydrocarbons.Since these mixtures, however, are

hardly analyzable at present and usual ring analysis methods are notapplicable, an accurate 'and detailed structure of these compounds hasnot been determined.

A four naphthenic rings, and is entirely new.

The general characteristics of the polycyclic naphthenic hydrocarbon areas follows:

Specific gravity d 0.85-0.99

Refractive index n l.45l.60

Viscosity (30C) cst 10-40 Hydrogen/carbon atomic ratio (hereinafterindicated by H/C) 1.75-2.00 Ignition point 1 25C Pour point 30 to 60CDielectric constant (a) (C) Dielectric tangent tan 8 (80C) 0.00l

Volumatic resistivity (fl-cm) (80C l0 Insulation breakdown voltageKV/2.5mm 60 Total acid value mg, KOH/g 0.00l

Corrosivity' l A Thermal stability (C-75 hrs) Sludge 0.20

Total acid value mg, KOH/g 0.20

However, this conventional oil has an inferior gas absorptivity.According to the present invention, this disadvantage has been overcomeby the addition of certain polycyclic aromatic compounds to thepolycyclic naphthenic hydrocarbon without adversely affecting itsinherent electrical properties. g

The polycyclic aromatic hydrocarbon employed as the other component inthe oil of the present invention includes polycyclic aromatics havingfrom 2 to 4 rings such as, for example, naphthalene, diphenyl,acenaphthene, fluorene, terphenyl, pyrene, chrysene, and C or lower,alkyl derivatives thereof, and mixtures of two or more such compounds.In addition, an arc matic hydrocarbon in which a portion of the ring issaturated with hydrogen, such as acenaphthene, can be used withoutdeparting from the scope of the present invention.

The aforesaid aromatic mixture, however, cannot be completely analyzed,but after various physical analysis, gas chromatographic analysis, etc.,the polycyclic aromatic structure was confirmed.

The polycyclic aromatic hydrocarbon utilized in the present inventionhas the following characteristics:

Specific gravity d) 1.00 1.20 Refractive index n, L50 1.68 Viscosity(30C) est 10 25 Pour point 40C Ignition point 1S0C The oil compositionof the present invention exhibits the following electrical propertiesand thermal stability:

Dielectric constant (s) (80C) 2.35 2.65 Dielectric tangent tanS (80C)0.001 0.03 Volumatic resistance (fl-cm) (80C) 2 X10 2 X10" Insulationbreakdown voltage KV/2.5mm 50 80 Thermal stability (125C 75 hrs) Sludge0.008 0.40 Total acid value 0.05 0.60

Due to its aromatic structure, the oil of the present invention has anexcellent gas absorptivity as well as excellent electrical properties.

The particular polycyclic naphthenic hydrocarbons and the polycyclicaromatic hydrocarbons used in the present invention are easilyobtainable by desulfurizing and alkylating (with lower olefins) and/ornuclear hydrogenating bottom oils or tarry materials rich in aromaticcompounds, the latter type of material is, for example, formed duringthe thermal cracking of petroleum hydrocarbons (crude oil, heavy oil,light oil, kerosene, naphtha and the like petroleum fractions) at atemperature of above 700C but below 2,300C for a period of time of1-0.001 second to produce ethylene and/or acetylene, or an oil tar fromthe gasification of heavy or crude oil at a high temperature, or coaltar, or bottom oil from the dealkylation of alkylaromatics (e.g., thebottom oil from toluene dealkylation during benzene production).

The desulfurization is conducted employing conventional reactionconditions and conventional catalysts at a temperature of 350-450C, apressure of 20-100 Kg/cm a hydrogen/high aromatic oil molar ratio of5-20, and an LHSV of 0.5-2.0. The catalyst which may be employedincludes cobalt-molybdenum, nickelmolybdenum, nickel-cobalt-molybdenum,etc., which may be supported on alumina or silica-alumina.

The alkylation is carried out in the presence of a catalyst by admixinga tarry fraction with an olefin gas.

The reaction conditions to be employed are as follows:

Reaction temperature 250 380C Pressure 1 50 Kg/crn Olefin/tar (molarratio) 0.2 Liquid hourly space velocity (LHSV) 0.1 -3.0

The catalyst employed in the alkylation may be an acidic catalyst suchas silica-alumina, zeolite, etc., but the use of a catalyst in which aGroup lll-B metal, such as lanthanum, cerium or thorium, is supported onzeolite is especially effective. Under these conditions, the alkylationproceeds smoothly to give the desired product in a good yield.

The degree of alkylation can be controlled to the desired extent bymodifying the olefin/tar mixing ratio or liquid hourly space velocity(LHSV).

The electrical properties of the product are not sig nificantlyinfluenced by the type of alkyl group introduced as long as it is notlarger than C Thus, the specific alkyl group employed is determinedbased on economics, considering the availability of the material. Thedegree of alkylation is suitably controlled, in

general, to such an extent that the number of alkyl groups attached peraverage molecular weight of the material aromatic oil ranges on theorder of below 5, preferably 1-4, since no substantial fluctuation inthe electrical peroperties of the product can be noticed within thisrange. A degree of alkylation departing from this range is not desirablefrom the viewpoint of the yield rather than the electrical properties ofthe product.

The hydrogenation reaction is conducted under conventional conditionsemployed in the art. Preferred hydrogenation catalysts to be used areexemplified by the oxides, sulfides, etc., of Group VI, VII and VIIImetals of the Periodic Table. These catalysts may be supported oncarriers such as activated carbon fullers earth, diatomaceous earth,bauxite, pumice stone, silica-alumina, etc. The reaction is ordinarilyconducted at a temperature of 100-450C, under a pressure of 10-300 kglcmat a liquid hourly space velocity (LHSV) of 0.5-2.0, and at ahydrogen/alkylated high aromatic oil molar ratio of 5-20.

The polycyclic naphthen s and polycyclic aromatics should be mixed,according to the present invention, in most cases within a range of -20percent, by volume, of the former to 5-80 percent, by volume, of thelatter, although this ratio varies to some extent depending upon thetype of electrical equipment to be used. An increased proportion ofpolycyclic naphthenes beyond the above stated range will result in adecrease in gas absorptivity, and a proportion less than the statedrange will cause poorer electrical properties.

By mixing both components within the above-mentioned ratio, it ispossible to attain remarkable improvements, especially with respect tothe gas absorptivity and the voltage pressure resistance among otherelectrical properties, while still maintaining suitable control of theother properties such as tan 8, insulation resistance etc., toappropriate values for given uses. It is surprising that the thermalstability of the mixture in the presence of oxygen can be greatlyimproved to an extent far superior to that of either of the polycyclicnaphthenes or polycyclic aromatics individually.

Although each component of the hydrocarbon oil of the present inventionitself exhibits excellent electrical performance, it has now been foundthat surprising effects, which cannot be attained by the use of theconventional mineral type-insulating oil, have been realized by mixingboth compounds together.

The insulating oil obtained according to this invention has demonstratedoutstanding and excellent results, especially when used as a transformeroil, a cable oil and a condenser oil having thermal resistance.

The present invention will be further explained by reference to thefollowing non-limiting examples.

EXAMPLE 1 A tarry material obtained by treating Seria crude oil at 1250Cfor a contact time of 0.005 second was determined by analysis to have anaromatic distribution comprising 47 percent dicyclic compounds, 33percent tricyclic compounds, 19 percent tetracyclic compounds and 1percent pentacyclic compounds, and determined by NMR to have a total of7 percent side chain hydrogen comprising 4 percent CH and 3 percent CHThis tarry material was desulfurized in a hydrogen gas stream by usingan ordinary desulfurization catalyst consisting ofcobalt-molybdenum-alumina, then alkylated with ethylene usingsilicaalumina as a catalyst to give a polycyclic aromatic hydrocarbonEXAMPLE 2 A bottom oil material was obtained as a by-product in theproduction of ethylene-propylene by the thermal cracking of Kuwaitnaphtha at 800C for the contact 5 mlxture time of 0.5 second. Thisbottom oil had an aromatic The mixture had bolhong P (calfillllated atring distribution comprising 76 percent dicyclic comnormal P 280 pQ l Pafter pounds, 21 percent tricyclic compounds and 3 percent type analysisby silica gel absorption comprising 99.1 tetracyclic compounds, andtotal side chain hydrogen weight percent aromatics and 0.9 weightpercent nonof 20 percent comprising 5% CH 5% CH and 10% aromatics, and,by Mass spectrometry analysis, com- CH --Cl-l by NMR analysis, which wasan aromatic oil prising 60.1 percent dicyclic rings, 35.4 percentcomalniflg relatively y Side Chaiflstricyclic rings and 4.5 percenttetracyclic rings. This oil was desulfurized in a manner similar to thatA portion of (B) was completely nuclearshown in Example 1 to prepare apolycyclic aromatic hydrogenated using a conventional nickel-aluminatype hydrocarbon t ,7 catalyst to form a polycyclic naphthenichydrocarbon had the followmg Propemesi bollmg P l' mixture culated atnormal pressure) 280-380C, a composi- (A) had the following properties:boiling point (cal- P 9? tyPehanalyss by 5 wfw culated at normalpressure) 290-370C, a composi- L 6 P :5 i a t tion after type analysisby silica gel absorption comprise mma a y Spe.c tome ry ana in 2 6 Weiht cream aromatics and 97 4 Wei ht er comprising 70.5 percent dicyclicrings, 26.8 percent g g M g tricyclic rings and 2.7 tetracyclic rings.gem wf' y t speftmmgtry ana This aromatic hydrocarbon mixture (D) was 9Pu e P dlcychc g Percent hydrogenated according to a procedure similarto that W and percent fetrafychc "P described in Example 1 to prepare apolycyclic (A)and (B) where then mixed in a ratio of 75/25, naphthenichydrocarbon mixture 50/50 and y Volume, p (C) had the followingproperties boiling point (cai- The properties and electricalperformances of the culated at normal pressure) =2g0 330 a i. resultingcomposlllons are given Table A 15 tion after type analysis by silica gelabsorption comprisapparent from the table, the characteristics of thesein 1,8 wei htnt aromatics d 93,2 weight mixtures after degradation werefound to be very excelcent non-aromatics; and by Mass spectrometryanalysis lent. comprising 44.2 percent dicyclic rings, 51.8% tricyclicFfl'fl' "V A n iiami Feed oil A Feed oil B Thermally stable insulatingoil I IS No. 2 oil (polycyclic (polycyclic (commerical naphthenes)aromatics) I(A/B =75/25 II(A/B =/50 III(A/B =25/75 mineral oil) Specificgravity d4" i 0. 983 1.002 0. 093 0. 99s 1. 002 0. 880 Retractiveindeiagnguu 1.2203 IigBg 1.1823 lig tg 1.1268 1.1332 Vimsity F- {wr' CI"514 410 414 413 411 314 Pour point, 0.-.. -42.5 010 42.5 -40.0 -45. 0-47.5 Ignition point, o 161 104 162 163 103 140 Acidic value mg., K H0.003 0.003 0. 003 0.003 0. 003 0. 004 Copper plate corrosion, angstroms1 1 1 1 1 1 Dieleetr c constant (80 C.) 2. 25 2. 2. 35 2.40 2. 53 2. 25tan a (80,) {Initial 0.001 0.07 0.01 0. 01a 0. 034 0.13 Afterdegradation 0.20 0. 86 0. 08 0. 04 0. 08 12.2 Initial 2. 0x10 00x10aoxio 1.3x10 20x10 50x10 Volumatlc resistance (80 C.)(ncm.) Afterdegl'fldationn 1. 0x10 4. 0x10 2. 5x10 4. 0x10 2. 3x10 2. 0x10 Hydrogengas absorptivity (111m) +15 180 -160 TABLE 2 M Feed oil 0 Feed oil DThermally stable insulating oils (polycyclic (polycyclic naphthene's)aromatics) IV(C/D=7fil25) V(C/D=50/50) VI(ClD=25/75) Specific gravity,di" 0. 942 1. 024 0.065 0.983 1.001 Refractive indexfio okn 1.19021.2833 1. 1.1382 1 .0 many c s. 2 4.3 5.0 4. s 4. 4 Pour point. C -43. 045. 0 43. 5 44. 5 45. 0 Ignition point, C 151 165 152 152. 5 153 Acidvaliie Ing.. KOH/g 0.003 0.003 0.003 0.003 0.003 Copper plate corroslonangstroms 1 l 1 1 1 Dielectric constant (8% 331.... 02b}; 0262; 2. :02655 n a... 0.00 0. 25 5 (Pelcem) {Alter degradation. 0. 20 0. 35 0. 070. 03 0. 000 Volumatic resistance {Initial 4.3)(10 1.0)(10 3.5)(104.8)(10 6. 0x10 15 C.) (ii-cm.) After degradation... 2. 5X10 8. 0X107.8)(10 5.0)(10 14 23x10" Hydrogen gas absorptlvity (mm.) 180 40 120 Acopper coil and a 100cc sample of the oil were placed in a 200cc beaker.and the degradation test was carried out for 75 hours in a hot air dryerat l20C.

"' A voltage of IOKV was applied at 80C under a hydrogen atmosphere, andthe gas generation and gas absorption were expressed by an oilmanometer.

rings and 4.0 percent tetracyclic rings.

Both hydrocarbon mixtures (C) and (D) were blended in ratios of 75/25,50/50 and 25/75 by volume,

respectively, to form three examples of the insulating oil of thisinvention, the properties and electrical per- See the note in Table Seethe note in Table 1.

EXAMPLE 3 The material oil used in this example was a byproduct bottomoil formed in a process for the production of ethylene and propylenewere Kuwait naphtha was thermally cracked in a tubular furnace at areaction temperature of 830C for the contact time of 0.4 second. Thisbottom oil was hydrogenated using a nickel-molybdenum-alumina catalystin the presence of hydrogen at a temperature of 350C, an LHSV of 0.8 andunder a pressure of 40 Kglcm G. The resulting hydrogenated product wasthen reacted with 1.5 mole equivalents of propylene in the presence of asilica-alumina type catalyst under apressure of 10 Kg/cm, an LHSV of 1.0and at a temperature of 200C. The oily product was distilled underreduced pressure to give a fraction (E) boiling in the range of 300-350Cin a yield of 52 percent. This fraction was then determined by massspectrometry and type analysis by adsorption on silica gel to have thefollowing composition:

Type analysis by Silica gel adsorption:

Aromatics weight 98. Non-aromatics weight 1.

Mass spectrometry Dicyclic rings Tricyclic rings Tetracyclic rings TABLE3 Thermally stable Thermally stable insulating oil insulating oil Vll(E/C-l-75/25 Vll (E/C=5l50) Specific gravity d, 0.982 0.961 Refractiveindex n,, 1.570 1.548 Viscosity (cst) 30C 16.3 17.2 75C 4.0 4.1 Pourpoint C 47.5 47.5 ignition point C 151 152 Acid value mgKOH/g 0.0020.002 Copper plate corrosion 1 A 1 A Dielectric constant (80C) 2.38 2.32tan 8 (80C) Initial 0.029 0.024

After degradation 0.05 0.04 Initial 1.3 X 10 9.5 X 10 Volumatic resis-tance (Q-cm) (80C) After degradation 4.8 X 10 3.7 X 10 Hydrogen gasabsorptivity (mm) l00 60 See the note in Table 1. See the note in Table1.

What is claimed is:

l. A thermally stable electrical insulating oil with improved gasabsorption performance comprising from 5 to 80 percent by volume ofamember selected from the group consisting of one or more polycyclicnaphthenic hydrocarbons, lower alkyl derivatives thereof and a mixtureof such naphthenic hydrocarbons and lower alkyl derivatives, and from 95to percent by volume of a mem lle selected from the group consisting ofone or more polycyclic aromatic hydrocarbons, lower alkyl derivativesthereof and a mixture of such aromatic hydrocarbons and lower alkylderivatives, the polycyclic naphthenic hydrocarbons being obtained byhydrogenating an aromatic fraction which is obtained by the thermalcracking of petroleum hydrocarbons at a temperature above 700C.

2. The thermally stable insulating oil of claim 1 wherein the one ormore polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereofand mixtures thereof, have a specific gravity d of from 0.85 to 0.99, arefractive index n of from 1.45 to 1.60, a viscosity (30C.) of from 10to 40 cst, a hydrogen/carbon atomic ratio of from 1.75 to 2.00, anignition point of above 125C., and a pour point of from --30 to 60 C.

3. The thermally stable insulating oil of claim 1 wherein the one ormore polycyclic aromatic hydrocarbons, lower alkyl derivatives thereofand mixtures thereof, have a specific gravity d of from 1.00 to 1.20, arefractive index n of from 1.50 to 1.68, a viscosity (30C.) of from 10to 25 est, an ignition point of above 150C., and a pour point of below-40C.

4. The thermally stable insulating oil of claim 1 wherein more than onepolycyclic naphthenic hydrocarbon is present as a mixture of polycyclicnaphthenic hydrocarbons, each having from two to five rings, their loweralkyl derivatives or a mixture thereof.

5. The thermally stable insulating oil of claim 1 wherein the polycyclicaromatic hydrocarbon is a mixture of polycyclic hydrocarbons each havingfrom two to four rings, their lower alkyl derivatives or a mixturethereof.

6. The thermally stable insulating oil of claim 1 wherein the polycyclicnaphthenic hydrocarbons, lower alkyl derivatives thereof and mixturesthereof are obtained by desulfurization, alkylation and hydrogenation ofan aromatic fraction which is obtained by the high temperature crackingof petroleum hydrocarbons.

7. The thermally stable insulating oil of claim 6 wherein the alkylationis carried out with butylene, propylene or ethylene.

8. The thermally stable insulating oil of claim 1 wherein the polycyclicaromatic hydrocarbons are obtained by desulfurization and alkylation ofan aromatic fraction which is obtained by the high temperature thermalcracking of the petroleum hydrocarbons.

9. The thermally stable insulating oil of claim 8 wherein the alkylationis carried out with butylene, propylene or ethylene.

10. The thermally stable insulating oil of claim 1 wherein the alkylportion of the lower alkyl derivatives contains less than eight carbonatoms.

11. The thermally stable insulating oil of claim 10 wherein the alkylportion of the alkyl derivative contains four or less carbon atoms.

12. The thermally stable insulating oil of claim I having the followingproperties:

Dielectric constant (a) (C) 2.35 2.65

Dielectric tangent tan 8 (80C) 0.001 0.03

Volumatic resistance (Q-cm) (80C) 2 X 10 2 X Insulation breakdownvoltage KV/2.5mm 50 80 Thermal stability (C 75 hrs) Sludge 0.008 0.40

Total acid value 0.05 0.60.

13. The thermally stable insulating oil of claim 1 wherein thepolycyclic hydrocarbons are obtained by desulfurization and alkylationof an aromatic fraction obtained by the high temperature cracking of thepetroleum hydrocarbons at a temperature greater than 700C. to yield thepolycyclic aromatic hydrocarbons, and a portion of the polycyclicaromatic hydrocarbons is thereafter hydrogenated to' yield thepolycyclic naphthenic hydrocarbons, the polycyclic naphthenic and thepolycyclic aromatic hydrocarbons thereafter being blended to yield thethermally stable insulating oil.

14. The thermally stable insulating oil of claim 13 wherein the thermalcracking is at a temperature of above 700C but below 2,300C for a periodof time above from 1 to 0.001 seconds.

- 15. The thermally stable insulating oil of claim 14 wherein thematerial cracked is selected from the group consisting of crude oil,heavy oil, light oil, kerosene, naphtha, an oil tar from thegasification of heavy or crude oil at a high temperature, a coal tar ora bottom oil from the dealkylation of alkyl aromatics.

16. The thermally stable insulating oil of claim 1 wherein thepolycyclic naphthenic hydrocarbon is selected from the group consistingof decaline, perhydrophenanthrene, perhydropyrene, perhydrofluorene,perhydroanthracene and perhydrochrysene, alkyl derivatives thereofwherein the alkyl group contains eight or less carbon atoms, andmixtures thereof.

17. The thermally stable insulating oil of claim 1 wherein thepolycyclic aromatic hydrocarbon has from 2 to 4 rings and is selectedfrom the group consisting of naphthalene, diphenyl, acenaphthene,fluorene, terphenyl, pyrene, chrysene, alkyl derivatives thereof whereinthe alkyl group has eight or less carbon atoms and mixtures thereof. I

1. A thermally stable electrical insulating oil with improved gasabsorption performance comprising from 5 to 80 percent by volume of amember selected from the group consisting of one or more polycyclicnaphthenic hydrocarbons, lower alkyl derivatives thereof and a mixtureof such naphthenic hydrocarbons and lower alkyl derivatives, and from 95to 20 percent by volume of a member selected from the group consistingof one or more polycyclic aromatic hydrocarbons, lower alkyl derivativesthereof and a mixture of such aromatic hydrocarbons and lower alkylderivatives, the polycyclic naphthenic hydrocarbons being obtained byhydrogenating an aromatic fraction which is obtained by the thermalcracking of petroleum hydrocarbons at a temperature above 700*C.
 2. Thethermally stable insulating oil of claim 1 wherein the one or morepolycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof andmixtures thereof, have a specific gravity d415 of from 0.85 to 0.99, arefractive index nD20 of from 1.45 to 1.60, a viscosity (30*C.) of from10 to 40 cst, a hydrogen/carbon atomic ratio of from 1.75 to 2.00, anignition point of above 125*C., and a pour point of from -30* to -60*C.3. The thermally stable insulating oil of claim 1 wherein the one ormore polycyclic aromatic hydrocarbons, lower alkyl derivatives thereofand mixtures thereof, have a specific gravity d415 of from 1.00 to 1.20,a refractive index nD20 of from 1.50 to 1.68, a viscosity (30*C.) offrom 10 to 25 cst, an ignition point of above 150*C., and a pour pointof below -40*C.
 4. The thermally stable insulating oil of claim 1wherein more than one polycyclic naphthenic hydrocarbon is present as amixture of polycyclic naphthenic hydrocarbons, each having from two tofive rings, their lower alkyl derivatives or a mixture thereof.
 5. Thethermally stable insulating oil of claim 1 wherein the polycyclicaromatic hydrocarbon is a mixture of polycyclic hydrocarbons each havingfrom two to four rings, their lower alkyl derivatives or a mixturethereof.
 6. The thermally stable insulating oil of claim 1 wherein thepolycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof andmixtures thereof are obtained by desulfurization, alkylation andhydrogenation of an aromatic fraction which is obtained by the hightemperature cracking of petroleum hydrocarbons.
 7. The thermally stableinsulating oil of claim 6 wherein the alkylation is carried out withbutylene, propylene or ethylene.
 8. The thermally stable insulating oilof claim 1 wherein the polycyclic aromatic hydrocarbons are obtained bydesulfurization and alkylation of an aromatic fraction which is obtainedby the high temperature thermal cracking of the petroleum hydrocarbons.9. The thermally stable insulating oil of claim 8 wherein the alkylationis carried out with butylene, propylene or ethylene.
 10. The thermallystable insulating oil of claim 1 wherein the alkyl portion of the loweralkyl derivatives contains less than eight carbon atoms.
 11. Thethermally stable insulating oil of claim 10 wherein the alkyl portion ofthe alkyl derivative contains four or less carbon atoms.
 12. Thethermally stable insulating oil of claim 1 having the followingproperties: Dielectric constant ( epsilon ) (80*C) 2.35 - 2.65Dielectric tangent (%) tan delta (80*C) 0.001 - 0.03 Volumaticresistance ( Omega -cm) (80*C) 2 X 1014- 2 X 1016 Insulation breakdownvoltage KV/2.5mm 50 - 80 Thermal stability (125*C - 75 hrs) Sludge %0.008 - 0.40 Total acid value 0.05 - 0.60.
 13. The thermally stableinsulating oil of claim 1 wherein the polycyclic hydrocarbons areobtained by desulfurization and alkylation of an aromatic fractionobtained by the high temperature cracking of the petroleum hydrocarbonsat a temperature greater than 700*C. to yield the polycyclic aromatichydrocarbons, and a portion of the polycyclic aromatic hydrocarbons isthereafter hydrogenated to yield the polycyclic naphthenic hydrocarbons,the polycyclic naphthenic and the polycyclic aromatic hydrocarbonsthereafter being blended to yield the thermally stable insulating oil.14. The thermally stable insulating oil of claim 13 wherein the thermalcracking is at a temperature of above 700*C but below 2, 300*C for aperiod of time above from 1 to 0.001 seconds.
 15. The thermally stableinsulating oil of claim 14 wherein the material cracked is selected fromthe group consisting of crude oil, heavy oil, light oil, kerosene,naphtha, an oil tar from the gasification of heavy or crude oil at ahigh temperature, a coal tar or a bottom oil from the dealkylation ofalkyl aromatics.
 16. The thermally stable insulating oil of claim 1wherein the polycyclic naphthenic hydrocarbon is selected from the groupconsisting of decaline, perhydrophenanthrene, perhydropyrene,perhydrofluorene, perhydroanthracene and perhydrochrysene, alkylderivatives thereof wherein the alkyl group contains eight or lesscarbon atoms, and mixtures thereof.
 17. The thermally stable insulatingoil of claim 1 wherein the polycyclic aromatic hydrocarbon has from 2 to4 rings and is selected from the group consisting of naphthalene,diphenyl, acenaphthene, fluorene, terphenyl, pyrene, chrysene, alkylderivatives thereof wherein the alkyl group has eight or less carbonatoms and mixtures thereof.